A door closer is typically attached to a hinged door such as a front door, fire door, etc. of a house, apartment, officetel, etc. so as to automatically close the door by elastic force when it is opened. In a typical door closer, when the opening angle of the door increases, the expansion of a spring increases, and thus the elastic force increases. Due to the nature of this structure, when the door is almost closed as the door closer operates, the spring is restored to its original length from the state where it is fully expanded, which decreases the elastic force, and thus it is likely that the door is not completely closed.
In order to cope with this problem, door closers provided with a unit for adding door-closing force in a state where the door is almost closed are disclosed in prior art documents such as, for example, Japanese Patent No: 2881552, Korean Patent No: 823143, Korean Patent No: 823854, etc. However, the unit for adding door-closing force is attached to the outside of a housing of the door closer, which increases the size and weight of the door closer and the risk of failure.